Stream Ciphering

1. A method of ciphering source data into target data, the method comprising: determining a level of ciphering for the source data; generating a key sequence depending on the determined level of ciphering; wherein the level of ciphering comprises or is indicative of a key entropy of the key sequence; wherein the key sequence comprises key symbols out of a key symbol alphabet; wherein the generating the key sequence comprises: generating a generator sequence according to a generator entropy that is greater than the determined key entropy; and mapping generator symbols of the generator sequence to key symbols of the key sequence; wherein the generator symbols are code words of a prefix code and bit lengths of the generator symbols are different; wherein the key entropy defines a ratio between an average bit length of the generator symbols and a bit length of the key symbols; and combining the source data and the key sequence resulting in the target data.

2. The method of claim 1 , wherein the determined key entropy for the key sequence is less than 1.

3. The method of claim 1 , wherein different bits in the key sequence are correlated depending on the determined key entropy and/or the determined level of ciphering.

4. The method of claim 1 , wherein different bits in the key sequence are differently probability-distributed depending on the determined key entropy and/or the determined level of ciphering.

5. The method of claim 1 , wherein a probability of a bit in the key sequence for being set to 1 deviates from ½ depending on the determined key entropy and/or the determined level of ciphering.

6. The method of claim 1 , wherein a cardinality of the key symbol alphabet and/or the bit length of the key symbols depends on the determined key entropy and/or the determined level of ciphering.

7. The method of claim 1 , wherein the key entropy is an increasing function of a cardinality of the key symbol alphabet and/or a decreasing function of the bit length of the key symbols.

8. The method of claim 1 , wherein the determined key entropy defines a ratio between a logarithm of a cardinality of the key symbol alphabet and the bit length of the key symbols.

9. The method of claim 1 , wherein a cardinality of the key symbol alphabet is less than 2 m, wherein m is the bit length of the key symbols.

10. The method of claim 1 , wherein the level of ciphering is changed by the determining without reseeding a generator generating the generator sequence.

11. The method of claim 1 , wherein the mapping is injective.

12. The method of claim 1 , wherein an average bit length of the generator symbols is less than the bit length of the key symbols.

13. The method of claim 1 , wherein the generator entropy is equal to or approximates a maximum entropy.

14. The method of claim 1 , wherein a length of a seed key for initializing the generating and/or a length of a period of the generating is not reduced when at least one of the level of ciphering or the key entropy is reduced.

15. The method of claim 1 , wherein the ciphering comprises encrypting or decrypting.

16. The method of claim 1 , wherein the source data, the target data, and/or a control message is indicative of the level of ciphering.

17. A non-transitory computer readable recording medium storing a computer program product for ciphering source data into target data, the computer program product comprising program instructions which, when run on processing circuitry of one or more computing devices, causes the one or more computing devices to: determine a level of ciphering for the source data; generate a key sequence depending on the determined level of ciphering; wherein the level of ciphering comprises or is indicative of a key entropy of the key sequence; wherein the key sequence comprises key symbols out of a key symbol alphabet; wherein the generating the key sequence comprises: generating a generator sequence according to a generator entropy that is greater than the determined key entropy; and mapping generator symbols of the generator sequence to key symbols of the key sequence; wherein the generator symbols are code words of a prefix code and bit lengths of the generator symbols are different; wherein the key entropy defines a ratio between an average bit length of the generator symbols and a bit length of the key symbols; and combine the source data and the key sequence resulting in the target data.

18. The non-transitory computer readable recording medium of claim 17 , wherein an average bit length of the generator symbols is less than the bit length of the key symbols.

19. A device for ciphering source data into target data, the device comprising: processing circuitry; memory containing instructions executable by the processing circuitry whereby the device is operative to: determine a level of ciphering for the source data; generate a key sequence depending on the determined level of ciphering, wherein the level of ciphering comprises or is indicative of a key entropy of the key sequence, wherein generating the key sequence comprises: generating a generator sequence according to a generator entropy that is greater than the determined key entropy; and mapping generator symbols of the generator sequence to key symbols of the key sequence; wherein the generator symbols are code words of a prefix code and bit lengths of the generator symbols are different; wherein the key entropy defines a ratio between an average bit length of the generator symbols and a bit length of the key symbols; and combine the source data and the key sequence resulting in the target data.

20. The device of claim 19 , wherein an average bit length of the generator symbols is less than the bit length of the key symbols.